Blender transforming quads to tris
The problem is most likely caused by the fact your vertices don't align well on the plane of the quad. And the render engine doesn't magically create new data and uses the quad as is ( which is basically just 2 triangles anyways ). If you look at the lower vertex of the quad you can see it's pointing a bit inwards. If you were to divide a quad into 2 triangles you'd see an edge going trough the middle and that's exactly what happens here.

You might want to consider adding more faces to accommodate for slope or change your shape a bit to make the quads vertices more aligned. Or use smooth shading and the normals of the face are interpolated to bend between faces.

Technically @Ween 's answers should work as it aligns the vertices to be on the same plane preventing the triangle bend of the quad.